Inks for 3D printing gradient refractive index (GRIN) optical components

ABSTRACT

Optical inks suitable for 3D printing fabrication of gradient refractive index (GRIN) optical components are composed of a monomer matrix material [ 100]  in which ligand-functionalized nanoparticles [ 102]  are well dispersed at more than 2% loading to induce a change in the index of refraction of the matrix of at least 0.02. The ligands are less than 1.2 nm in length and are covalently bonded to both the nanoparticles and the monomer matrix. The nanoparticles are less than 100 nm in size and the doped matrix material has a transmittance of at least 90% at wavelengths of interest. The matrix material has less than 20 cPoise viscosity and is UV crosslinkable to form a cured polymer.

FIELD OF THE INVENTION

The present invention relates generally to optical ink compounds. Morespecifically, it relates to inkjet-printable optical ink compositionssuitable for 3D printing of gradient refractive index (GRIN) opticalcomponents.

BACKGROUND OF THE INVENTION

Gradient refractive index (GRIN) optical structures are composed of anoptical material whose index of refraction, n, varies along a spatialgradient in the axial and/or radial directions of the lens. They havemany useful applications such as making compact lenses with flatsurfaces.

There are several known techniques for fabricating GRIN lenses. Oneapproach is to press films of widely varying refractive indices togetherinto a lens using a mold, e.g., as taught in U.S. Pat. No. 5,689,374.This process, however, is expensive to develop. A second approach forfabricating GRIN lenses is to infuse glass with ions at varying density.This approach has reached commercial production, but it is alsoexpensive and effectively limited to small radially symmetric lenses bythe depth to which ions will diffuse into glass. A third approach forfabricating GRIN lenses is to use 3D printing technology with inkscomposed of a polymer matrix doped with particles which change the indexof refraction of the matrix. Each printed droplet has a distinctrefractive index controlled by the concentration of dopants in thepolymer material. This approach is described, for example, in R.Chartoff, B. McMorrow, P. Lucas, “Functionally Graded Polymer MatrixNano-Composites by Solid Freeform Fabrication”, Solid FreeformFabrication Symposium Proc., University of Texas at Austin, Austin,Tex., August, 2003, and in B. McMorrow, R. Chartoff, P. Lucas and W.Richardson, ‘Polymer Matrix Nanocomposites by Inkjet Printing’, Proc. ofthe Solid Freeform Fabrication Symposium, Austin, Tex., August, 2005.

Although using 3D printing has the potential to provide an efficient andinexpensive means of fabricating GRIN lenses, a number of unsolvedproblems have prevented or significantly limited its practicalrealization. One of the most significant problems is that the inkcompounds need to simultaneously have all the desired properties forhigh quality GRIN lenses while at the same time need to have propertiessuitable for 3D printing using inkjet technology. In particular, it isimportant that the doping of the host matrix creates a substantialchange in the index of refraction of the host matrix, so that the GRINlens can efficiently provide significant optical power. It is alsoimportant that the material, both when doped and undoped, besubstantially transparent at wavelengths of interest (e.g., visiblespectrum) so that light is transmitted through the lens rather thanabsorbed. At the same time, in order to be suitable for the 3D printingprocess, the ink material must have a low viscosity both with andwithout doping, and be curable by a process that does not createuncontrollable distortion of the printed lens. Despite the desirabilityfor an ink satisfying all of these criteria, researchers have yet tounderstand what physical characteristics of matrix and dopant materialsare sufficient to produce inks satisfying all these properties, or todiscover any specific ink compounds that simultaneously possess allthese properties. As a result, the realization of 3D printing of highquality GRIN lenses remains elusive.

SUMMARY OF THE INVENTION

The present inventors have clearly specified for the first time the keyphysical characteristics of matrix materials and dopants that aresufficient to provide all the important properties suitable for 3Dprinting of high quality GRIN lenses. They have also discovered anddescribed herein a variety of specific examples of such ink compounds.These inks have the following key physical characteristics. The matrixmaterial is a monomer that is UV crosslinkable with 20% or less shrinkto minimize the strain and subsequent deformation of the opticalstructure. The matrix material has a transmittance of at least 90%(preferably at least 99%) at the wavelengths of interest, and theviscosity of the matrix in its monomer form is less than 20 cPoise sothat it can be inkjet printed. The matrix material is doped withnanocrystal nanoparticles at a loading of at least 2% by volume. Thenanocrystals are selected such that a difference in index of refractionbetween the doped and undoped matrix material is at least 0.02, i.e.,Δn≧0.02. The nanocrystal sizes are sufficiently small that they do notinduce Mie or Rayleigh scattering at the wavelengths of interest (e.g.,less than 50 nm in size for visible wavelengths, less than 100 nm for IRwavelengths). The nanocrystal material, as well as the doped matrixmaterial, preferably has a transmittance of at least 90% (morepreferably, at least 99%) in a predetermined optical wavelength range(e.g., visible spectrum). The nanocrystals are functionalized withligands selected to ensure that the nanocrystals are well dispersed inthe matrix. Specifically, each ligand is less than 1.2 nm in length, iscovalently bonded at its anchor end to the nanocrystal, and iscovalently bonded at its buoy end to the monomer so that good dispersionis maintained during polymerization. In addition, the buoy ends of theligands repel each other to help prevent aggregation and lightscattering, resulting in less than 5% scattering from aggregatednanoparticle clusters. These physical characteristics provide guidanceto those skilled in the art to identify a class of inks suitable for 3Dprinting of GRIN lenses. A number of examples of such inks are describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A: Cross-sectional schematic diagram of an optical ink composed ofa matrix material doped with nanoparticles, according to an embodimentof the invention.

FIG. 1B: Schematic diagram of a nanoparticle functionalized with aligand, according to an embodiment of the invention.

FIG. 2: Chemical structure of HDODA (1,6-hexanediol diacrylate).

FIG. 3: Scheme for an alternate synthetic route to metallic salts ofHHT.

DETAILED DESCRIPTION

Embodiments of the present invention include optical inks suitable foruse in fabricating GRIN lenses using 3D printing technology such asstandard drop-on-demand inkjet printing. These inks may also be used tofabricate GRIN lenses using other printing techniques such as screenprinting, tampo printing, aerosol jet printing, and laser cure printing.The optical inks prepared according to embodiments of the presentinvention are composed of a matrix material composed of a monomer andnanoparticles dispersed in the matrix material. The nanoparticle-dopedmonomer matrix material (a liquid) is placed in an inkjet printhead. Inaddition, an adjacent inkjet printhead is filled with undoped matrixmaterial. Additional printheads may also be filled with another opticalink with a different type or concentration of nanoparticle.Drop-on-demand inkjet printing technology is used to create microscopic,on-the-sample mixtures of the two (or more) liquids, thereby creatingprecisely-controlled and highly-localized regions having optical indexcontrolled by the mixture of the undoped and doped inks. The localizedcomposition and three dimensional structure is locked-in by polymerizingthe monomeric solution into an optical-quality polymer. Each droplet ofpolymer that is deposited onto the substrate on which the GRIN lens isbeing formed, can be created with a desired concentration ofnanoparticles. The volumetric concentration of nanoparticle within agiven droplet volume determines the effective refractive index of thatmaterial. Drop on demand printing, such as inkjet printing, allows forthe formation of three dimensional structures where different volumeswithin the structure contain different concentrations of dopants toeffectively change the refractive index within a three dimensionalstructure. The creation of precise three dimensional optical lenses andother optical structures by stereolithography is known to those skilledin the art of GRIN lens design. Embodiments of the present inventionprovide inks suitable for the practical realization of such 3D printableinks for high quality GRIN lens fabrication. These inks provide theability to control the index of refraction in three dimensions forcreating large, localized index changes while maintaining high opticaltransmission and freedom from deleterious scattering phenomena.

Since drop-on-demand inkjet may utilize multiple printheads withdifferent loading of the index-changing dopant, the inks provided by thepresent invention may be used in various combinations with each other aswell as with other optical inks. The ability to alter, in threedimensions, the index of refraction both above and below that of thehost matrix opens the design space for GRIN optical components.

According to embodiments of the present invention, an optical ink iscomposed of a matrix material 100 doped with nanoparticles 102, as shownin FIG. 1A. The matrix 100 is composed of a monomer that is capable ofbeing UV cured to create a solid polymer, and the nanoparticles 102 areeach functionalized with a ligand 104, as shown in FIG. 1B.

The matrix material 100 is a monomer that is UV crosslinkable.Preferably, the UV curing results in at most 20% shrink, which serves tominimize the strain and subsequent deformation of the optical structure.The viscosity of the matrix 100 in its monomer form is less than 20cPoise so that it can be inkjet printed. When cured, the matrix material100 preferably has a transmittance of at least 90% (preferably at least99%) at the wavelengths of interest (e.g., visible spectrum).

Dispersed within the matrix 100 are nanoparticles 102 which preferablyare nanocrystals. The nanoparticles 102 are present in the matrix 100 ata loading of at least 2% by volume, altering the index of refraction ofthe undoped matrix by at least 0.02, i.e., Δn≧0.02. In addition, inorder to preserve transparency of the ink, the nanoparticles 102 aremade sufficiently small that they do not induce Mie or Rayleighscattering at the wavelengths of interest. For example, for GRIN lensesdesigned to operate in the visible spectrum, the sizes of thenanoparticles 102 are less than 50 nm. For operation in the infraredspectrum, the sizes are less than 100 nm. In addition, also to helppreserve transparency, the nanoparticles 102 are preferably made ofmaterials that transmit greater than 90% (preferably greater than 99%)of the light in a predetermined optical wavelength range (e.g., visibleor infrared). In order to achieve more than 2% loading and large Δnwhile also providing a well-dispersed doping (and transparency), thenanoparticles 102 are functionalized with ligands 104 that are less than1.2 nm in length. In addition, each ligand 104 is covalently bonded atits anchor end to the nanocrystal 102, and is covalently bonded at itsbuoy end to the monomer material of the matrix 100. In addition, thebuoy ends of the ligands 104 preferably repel each other to help preventaggregation and light scattering, resulting in less than 5% scatteringfrom aggregated nanoparticle clusters. These physical characteristics ofthe ligands ensure that the nanoparticles 102 are well dispersed in thematrix and that good dispersion is maintained during polymerization.

These inks will now be further described and illustrated in the contextof several concrete examples. Those skilled in the art will appreciatethat the principles, teachings, and techniques discussed in thefollowing examples are not limiting but in fact provide furtherillustration of the range of possible inks that are encompassed withinthe scope of the invention.

According to one embodiment of the invention, the matrix or host polymeris 1,6-hexanediol diacrylate (HDODA), and the nanoparticle is anorganometallic compound. The organometallic compound may be, forexample, any of various salts of metals such as zinc (Zn²⁺), lead(Pb²⁺), titanium (Ti⁴⁺), and other metallic salts that are clear andtransparent. More generally, the metallic salt may have a cationconsisting of Ti⁴⁺, Pb²⁺, Zn²⁺, Al³⁺, Sn⁴⁺, In³⁺, Ca²⁺, Ba²⁺, Sr²⁺, Y³⁺,La³⁺, Ce³⁺, Nd³⁺, Pr³⁺, Eu³⁺, Er³⁺, Yb³⁺, Gd³⁺, Ho³⁺, Sm³⁺, Tb³⁺, Dy³⁺,Tm³⁺, Zr⁴⁺, Hf⁴⁺ or Ta⁵⁺.

Ligand functionalization of clear, transparent metallic salts providematrix compatibility with HDODA, allowing high density loading of theorganometallic salt into the matrix. Furthermore, due to a difference ofindex of refraction between undoped HDODA and HDODA doped with thefunctionalized metallic salt, GRIN lenses may be formed usingdrop-on-demand printing techniques such as inkjet printing. The metallicsalts interact favorably with a host matrix material such that greaterthan 90% transparency is obtained in the spectral region spanning 375 nmthrough 1600 nm.

As shown in FIG. 2, HDODA is a well-known material for the fabricationof clear coatings. It has a low viscosity (7.9 cP) making it amenabletowards dispensing using drop-on-demand techniques such as inkjetprinting. It also has a large spectral window in which greater than 99%transparency is observed, making it ideal to construct lensing material.HDODA also has an index of refraction of 1.456. By using anorganometallic compound having an index of refraction different fromthat of HDODA, using drop-on-demand techniques such as inkjet printing,gradient refractive index lenses may be fabricated having control of theindex of refraction in three dimensions.

In order for the nanoparticle to be successfully incorporated into anHDODA solution, it must have a ligand chemistry (surface treatment) thatis compatible with the HDODA. To do this, a ligand mimicking the hostmatrix material can be utilized to ensure that metallic salts can beincorporated into the HDODA without undergoing phase segregation,precipitation, or other means of separation from the host material.

As examples, an ink formulation may use a metal-ligand placed in1,6-hexanediol diacrylate at 1-75 wt % with 0.1-5.0 wt % Irgacure 184and 0.1-5.0 wt % Irgacure 819. Another ink formulation may use ametal-ligand placed in 1,6-hexanediol diacrylate at 1-75 wt % with0.1-5.0 wt % Irgacure 184 and 0.1-5.0 wt % Irgacure 819. Another inkformulation may use a metal-ligand placed in 1,6-hexanediol diacrylateat 1-75 wt % with 0.1-5.0 wt % Irgacure 184 and 0.1-5.0 wt % Irgacure819. Yet another ink formulation that may use a metal-ligand mixtureplaced in 1,6-hexanediol diacrylate at 1-75 wt % with 0.1-5.0 wt %Irgacure 184 and 0.1-5.0 wt % Irgacure 819.

A route to obtaining the material to print this lens uses the alcoholterminated ligands as shown in reaction sequence depicted in FIG. 3.

According to another embodiment, stereolithographic techniques are usedto print in 3D using droplets of the monomeric form of optical polymers(such as 1,6-Hexanediol diacrylate, aka HDDA or HDODA, or styrene) dopedwith varying levels of nanoparticles having an index refraction that issignificantly different from that of the optical polymer. With eachdroplet being able to deliver a distinct refractive index, thistechnology allows true 3D creation of GRIN optical elements of arbitraryshape. A challenge in the past has been the limited selection ofnanosized materials which are both transparent in the optical range ofinterest and have a significant difference in refractive index, ascompared to optical polymers whose refractive index typically fallsbetween that of polystyrene and of poly methyl methacrylate (1.592 and1.489 respectively). According to this embodiment, diamond is used asthe nanoparticle material. It has an optical window spanning from about0.25 microns to 80 microns and a refractive index of 2.42, which islarge compared to most optical polymers. Recently-developed diamondnanocrystals have dimensions less than one tenth the wavelength of light(i.e., less than or equal to 50 nm diameter particles for visiblelight), which minimizes Mie or Rayleigh scattering. Using drop-on-demandtechniques such as inkjet printing, gradient refractive index opticalcomponents may be fabricated having control of the index of refractionin three dimensions.

The diamond nanoparticles are first coated with a ligand material whichprovides chemical compatibility with the optical polymer, then blendedwith the monomeric form of the optical material, delivered using 3Dprinting technology, and finally polymerized into transparent solidswhich serve as GRIN optical structures. Arbitrarily 2D and 3D GRINoptical components are fabricated by drop-on-demand sterolithographicinkjet printing or by other printing techniques.

Nanodiamonds are commercially available as byproducts of refineries,mining operations and quarries and are broken down to smaller sizesthrough ball milling and sonication. Alternately detonation nanodiamond(DND), often also called ultradispersed diamond (UDD), is diamond thatoriginates from a detonation of an oxygen-deficient explosive mixture ofTNT/RDX. In both cases particles of 4-6 nm are obtained.

For the diamond nanomaterial to be successfully incorporated into anoptical polymer matrix, it is provided with a surface treatment (i.e.,ligand) that is compatible with the polymer. The ligand is described byanalogy with anchor-chain-buoy configuration in which the “anchor” endof the ligand molecule is the chemical entity that covalently binds tothe surface of the diamond nanoparticle, the “buoy” at the other end isthe entity which provides chemical compatibility with polymeric matrix,and the “chain” refers to the length of the linkage between the twopreviously mentioned entities. Carboxylates, amines, thiol groups mayprovide the anchor entity for bonding to the carbon-based nanodiamonds.The buoy group is selected to match the chemical nature host opticalpolymer, for example an acetylacrylate group for compatibility withacrylate-based polymers, and disperse the functionalized nanoparticlesby repelling other buoys. Minimal chain lengths of 2-4 carbon atoms areeffective for dispersing nanoparticles in the size range form 2-20 nm.

In a preferred embodiment, the appropriately-coated diamondnanoparticles are blended with the monomeric form of the optical polymerwith a specified percent loading. The nanoparticle-doped liquid isplaced an inkjet printhead, in tandem with an inkjet printheadcontaining pure monomer. This can be extended to multiple printheadswith different percent particle loading. Drop-on-demand inkjet printingtechnology is used to create microscopic, on-the-sample mixtures of thetwo (or more) liquids, thereby creating precisely-controlled andhighly-localized regions of variable optical index. The localizedcomposition and three dimensional structure is locked-in by polymerizingthe monomeric solution into an optical-quality polymer. This optical inkthus provides the ability to concurrently modify the index of refractionin three dimensions by use of the doped nanoparticles, providing themeans for creating large, localized index changes while maintaining highoptical transmission and freedom from deleterious scattering phenomena.

According to another embodiment of the invention, stereolithographictechniques are used to print in three dimensions using droplets of themonomeric form of optical polymers (such as 1,6-Hexanediol diacrylate,aka HDDA, or styrene) doped with varying levels of nanoparticles havingan index refraction that is significantly different from that of theoptical polymer. With each droplet being able to deliver a distinctrefractive index, this technology allows true 3D creation of GRINoptical elements of arbitrary shape. A challenge in the past has beenthe limited selection of nanosized materials which are both transparentin the optical range of interest and have a significant difference inrefractive index, as compared to optical polymers whose refractive indextypically falls between that of polystyrene and of poly methylmethacrylate (1.592 and 1.489 respectively). Although there are severaltransparent materials (such as TiO₂ and ZnS) with refractive indexsignificantly higher that of the polymer, there are very few materialswith refractive index significantly lower than 1.5. According to thisembodiment, microscopic hollow nanoparticles are used to effectivelyinsert transparent, low refractive index “air-bubble” dopants intooptical polymers for the fabrication of gradient refractive index (GRIN)optical components.

The index-changing hollow-sphere dopants are mixed with the monomericform of the optical polymer to create stock solutions. Drop-on-demandinkjet printing of the stock solution, followed by rapid polymerization,yields gradient refractive index optical components with control of theindex of refraction in three dimensions.

With air (or other gases) in their centers, the hollow microspheres havea broad optical transmission window and an (interior) refractive indexof 1.0, which is significantly smaller than that of most opticalpolymers. If the shell of the microsphere has a refractive index matchedto that of the optical polymer matrix, it is the just the interiordimension which needs to be less than or equal to one tenth thewavelength of light to avoid Mie or Rayleigh scattering. The hollownanoparticles may be created via a micro-emulsion technique, then coatedwith a ligand material which provides chemical compatibility with theoptical polymer, blended with the monomeric form of the opticalmaterial, and finally polymerized into transparent solids. Arbitrarily2D and 3D GRIN optical components are fabricated by drop-on-demandsterolithographic inkjet printing or by other printing techniques.

The nanoparticles in some embodiments may be hollow nanospheres or anycarbon nanostructure (tube, buckeyball, nanodiamond, etc.) with agaseous or empty core. Polymeric and inorganic-shelled hollowmicrospheres may be fabricated using micro-emulsion techniques, yieldingmicrospheres of sizes down to 100 nm. To minimize Mie or Rayleighscattering dimensions are preferably less than one tenth the wavelengthof light (e.g., at most 50 nm diameter particles for visible light). Ifthe refractive index of the shell material is matched to that of thepolymeric host, it is only the interior dimension that encompasses thelow index gas that needs to be at or below one tenth the wavelength oflight.

In order for the nanomaterial to be successfully incorporated into anoptical polymer matrix, it is provided with a surface treatment (usingligand chemistry) that is compatible with the polymer. Carboxylates,amines, thiol groups provide the anchor entity for bonding to thecarbon-based nanodiamonds. The buoy group is selected to match thechemical nature host optical polymer, for example an acetylacrylategroup for compatibility with acrylate-based polymers. Minimal chainlengths of 2-4 carbon atoms are effective for dispersing nanoparticlesin the size range form 2-20 nm.

The invention claimed is:
 1. An optical ink composed of a monomer matrixmaterial doped with ligand-functionalized nanoparticles; wherein themonomer has a viscosity less than 20 cPoise and is UV curable to form asolid polymer; wherein the matrix material doped with theligand-functionalized nanoparticles has a transmittance of at least 90%in a predetermined optical wavelength range, wherein theligand-functionalized nanoparticles have a size less than 100 nm, areloaded in the monomer matrix material at a volume percent of at least2%, and alter an index of refraction of the monomer matrix by at least0.02; wherein each of the ligand-functionalized nanoparticles iscovalently bonded to a ligand, where the ligand is covalently bonded tothe monomer matrix material, and has a length less than 1.2 nm.
 2. Theoptical ink of claim 1 wherein buoy ends of the ligands repel eachother.
 3. The optical ink of claim 1 wherein the ligand-functionalizednanoparticles have a size less than 50 nm.
 4. The optical ink of claim 1wherein the nanoparticles are ligand modified metallic salts,nanocrystal diamond, nanocrystal microspheres, or a hollow nanomaterial.5. The optical ink of claim 1 wherein the monomer matrix is composed of1,6-hexanediol diacrylate, or an acrylate optical polymer.